This invention relates to a composition and method for forming a zinc phosphate coating on a metal surface, particularly on iron and steel. The present invention is intended to provide a zinc phosphate coating solution which eliminates the use of any oxidant used conventionally for promoting the chemical conversion such as nitrate ion, nitrite ion, chlorate ion, bromate ion and the like. Absence of such oxidants serves to reduce the amount of sludge formed during the chemical conversion coating by reducing metallic ions dissolved from the base metal surface treated while imparting excellent corrosion resistance and other qualities.
Zinc phosphate coating has conventionally been applied on surfaces of iron and steel as a base coating for subsequent painting to improve the durability of the top coated paints. However, conventional zinc phosphate coating solutions contain, in general, an oxidant such as nitrate ion, nitrite ion, chlorate ion, bromate ion and the like as a promoter or accelerator. Such oxidants are decomposed during the chemical conversion coating generating undesirable by-product gases. Metals dissolved from the surface of the base metal during the chemical conversion coating contribute to sludge formation. The presence of an oxidant accelerates the rate of dissolution and also the rate of sludge formation. In addition, such oxidants are conventionally added in the form of alkali metal salts such as Na or K so that upon decomposition during the course of chemical conversion coating alkali metal ions accumulate in the bath. As a result, dihydrogen zinc phosphate is hydrolyzed to form sludge so that the rate of sludge formation tends to be increased even more. Such sludge must be removed and discarded periodically resulting in excessive labor and expense.
It has been disclosed in British Pat. No. 866,377, Japanese Patent Publication No. 26455/1967, 10925/1971 and 2648/1973 to add citric acid or tartaric acid to a zinc phosphate coating solution containing an oxidant. Such chemical conversion solutions contain an oxidant and free phosphoric acid and are controlled in such a manner that only when the contents of the phosphates and free phosphoric acid are maintained in a certain range, will a zinc phosphate coating be formed on the metal surface by adjusting the pH to a value of less than 3.5. Due to the inclusion of an oxidant, sludge is formed in an excessive amount. Disclosed in Prepublished Japanese Patent Application No. 5839/1974 is a treating solution containing predominantly an alkali metal phosphate and a metallic ion such as zinc in a minor amount ranging from 20 ppm to the solubility limit exhibiting a pH from 3.8 to 6.0. However, due to the fact that zinc phosphate has a lower solubility, the amount of dissolved zinc ion which can be maintained is low. (G. Chalot: Qualitative Analysis II translated in Kyoritsu Zensho, pp 432.) In addition, as the solubility decreases linearly with increased pH, such solution gives a substantially iron phosphate coating.